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GETTING THE BEST OUT ( MANUAL STEERING GEARS

5th November 1943
Page 26
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Page 26, 5th November 1943 — GETTING THE BEST OUT ( MANUAL STEERING GEARS
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Which of the following most accurately describes the problem?

IN order that accurate and light steering may be provided on a vehicle, the chassis designer must follow definite rule's. A knowledge of these rules is as important to the operator as to the designer, for it will enable him to keep the steering of his vehicles at it maximum efficiency.

In the first place the steering-box gear ratio must be such that, under the worst possible conditions, the manual effort required, to steer the vehicle does not exceed 50 lb. per hand. The ratio is often as low as 30 to 1 and in some cases, even a greater reduction is used Unfortunately, such big reductions necessitate a large number of turns of the steering wheel to move the road wheels from–lock to Lock, but, on heavy vehicles with purely manually operated steering, this is unavoidable and musf be accepted.

There must be no interference with the steering as a result of 'spring deflection. That is, the ball-end of the drop arm must be so located that the axle end of the drag link follows the motion of the axle steering arm, induced by the rise and fall of the wheels, as accurately as possible. Referringto Fig. 1, point A represents the virtual centre of oscillation of the spring about which point B can be said to revolve. In order to-determine the centre about which the axle steering-arm ball rotates, the .parallelogram A BC D must be constructed, C being the centre of the bail and D the centre of rotation.

If there is to be no interference, the drop-arm ball centre should coincide with point D but, in practice, this is impossible, as the angle between the drag link and the. axle steering arm, in plan, is too large on one lock, and too small on the other. Therefore, a compromise is necessary, the sleeting box being mounted SP that the drop-arm ball ceiltre is at point E on the Produced line joining C to D.

Were E at any other point, not on the line C D E, say, at point F, the interference would be greatly exaggerated as indicated by the different arcs—one in full line and the other in broken line (see Fig, 1.), whereas the arc described from point E varies so little from that desired (full line), that the very slight deviation can be ignored.

Why It Is Necessary to Avoid Spring Deflection Interference

Interference is extremely important for, were the steering boX of the ifreVersible type, the stress induced in the drag link, drop arm and axle. steering arm—by only a small difference in the paths—would be sufficient to.cause distortion if not actual fracture. With reversible steering, the whole force of the deflecting wheels would be passed back to the driver.

The centre distance, indicated in Fig. 2, must be a

reasonable dimension, say, between 1in. and ins., and this figure is determined by the king-pin and camber angles. The maximum value . of the "camber angle is 2 degrees, as any figure above this induces rapid tyre wear. so that the king-pin angle must be adjusted to provide the required centre distance, with the need for ample wheelbearing spread always being borne in Mind.

This latter point can prove to be something of a bugbear, for the larger the dimension between the bearings the nearer to the chassis centre-line is the king-pin moved and, cons sequently, the wider .becomes the king-pin angle. A wide king-pin angle is undesirable because; by increasing the height through which the front end of the vehicit is lifted when the wheels are turned, it tends to make the steering

heavy. .

On_ soine Service chassis, however, the-angle is as much as 12 degrees and, whilst the steering cannot be said to. be of the "finger-tip " type, when once a driver has got used

to it he dOes not suffer any undue 'fatigue on long journeys. In this case, however, the

wide angle is the result of front-wheel-drive

design, and sirch a high figure,should not be countenanced on a dead axle. A narrow

angle is sufficient to provide the necessary force to return the wheels to the straightahead position after they have been deflected, •

TO prevent wheel wobble, which is far more-likely to oecur on a vehicle with true,

centre-point steering than on one sv,..ith a centre distance as recommended above, and toLpssist in returning the wheels to the

s. aight-ahead position after cornering, a s.uitable castor angle must be chosen which

will provide a trail of about 1 in.

If the trail he excessive, the vehicle will wander from .sicle to side of the road and no amount of adjustment to the steering box or tightening of slatk fnints will cure it. It is important, then, that the degree of castor be accurate, for, on the one hand, if it be 'too small -there is a tendency to wheel wobble, and, on the other, if it be too large, it will cause the wheels to wander.

Wir...n a vehicle negotiates a corner, the two front wheels travel on circirlar paths of different radii and, therefore, their lock angles are different. As the turning circle decreases, the discrepancy between the lock angles increases, so thatthe relationship between the two is constantly varying, There is no steering system which ensures an absolutely accurate combination of angles on all locks, but the Ackermann principle is near enough for the inaccuracies to be ignored.

The principle 'depends entirely upon the-inclination of the trackrod arms to each other, the lines passing through the ends of the arms and the pivotal points of the wheels, to meet at a point on the centre line of the vehicle about one-quarter to one-third of the wheelbase from the rear axle. The actual position of this intersecting point decides whether the inaccuracy between the lock angles shall occur on full lock; mid-lock; or at a position just .off the straightahead.

It is possible, therefore, to arrange the steering of a vehicle to suit the type of work on which it is engaged. For instance, a bus or local-delivery vehicle, operating in a district demanding a great deal of falllock work, could be provided with steering which was accurate at both starting and full locks, but with inaccuracy at mid-lock. Were the steering incorrectly laid out, so that large differences occurred in the lock angles, rapid tyre weal and heavy steering would be unavoidable.

The 'final item to be determined in the steering layout is the amount of toe-in -required. It can be 'calculated by making allowances for wear in the track-rod joints, king-pin bushes and flexing of the track rod, but at best, the figure obtained is only an approximation. The most reliable figure is arrived at by experience, and is always available from the vehicle' maker. Incidentally, on a front-wheel-drive vehicle, what is normally "toe-in " becomes " toe-out," as the Wheels, being driven, tend to run around the king-pins, thus reducing the distance between them at the front.

The steering layout of twin-front-axle vehicles must obey all the above rules and, at -the same time, co-ordinate the action of the two axles to provide a steering system which shall be as near perfection as possible. The actual geometry of the linkage ignoring the connections to the steering box, is not difficult, for the chassis can be considered as two separate machines, the one superiniposed on the other, with their rear axles coinciding. .10 • 12 -14

In this way a perfect layout, giving the results depicted in Fig, 3, can be obtained. The point 0 is the instantaneous centre about which the three axles are rolling, and, by using the Ackermann principle, the wheels can he made to take. up the positions shown, One item must be watched, however, and that is the intersecting point of the: lines through the pivots and their respective track-rod arms. These points, P, and .132, in Fig. 3, should not coincide, for, were they so to do, the inaccuracies in wheel lock, which cannot be obviated,

.62 would occur at different positions of the wheels, so that constant interference between the turning circles as induced by the two steered axles, would manifest itself.

It is possible to calculate the angles of the arms to give a minimum interference, but the process is both lengthy and tiresome. The respective lock angles for all four wheels can be laid out on the drawing board to give the same results as the calculations, but this, too, requires a great deal-of time, as it is merely a case of trial and error until the most accurate solution is reached. Usually, a graph is employed, similar to that shown in Fig. 4, from which, after a decision has been made On the length of the track-rod arms, the angles can be read and the steering errors made to occur at equivalent lock angles for both axles.

Mechanical Considerations • Governing Correct Lock Angles

• -Whilst it is a comparatively simple matter to ensure that the four lock angles On the two axles can be co-ordinated to give a minimum of interference between their turning circles, it is by no means easy to arrange 'connections between the steering box and axle steering arms to give the desired 'results. When the vehicle is travelling in a straight line which, for purposes of argument must be considered as a circle of infinite radius, the wheels on the two steered axles travel on exactly the same paths, and the ratio of the turning radii induced is 1 to 1.

Were it possible for the vehicle to turn about the midpoint of the rear axle, the ratio would he the wheelbase measured to the front axle, divided by the wheelbase to Is the second axle or, roughly, Assuming a turning circle DA' OC of 65 ft.,' the ratio is — for the outer wheels and —

OB OD for the inner wheels, which, on a vehicle with a full wheelbase of 18 ft., and a secondary of 14 ft., are, roughly, 1.06 to 1 and 1.09 to 1 respectively. When the corner is left-hand the ratio will he 1.06 to 1, whilst a,, right-hand corner will demand the 1.09 to 1. ratio, as the outer wheels, in the former case, become the inner ones in the latter.

It will be appreciated, then, that to provide perfectly accurate steering, apart from the discrepancies due to the Ackermann principle, a lever system between the steering box and axles must be provided, which, for left-hand corners, varies between 1 to 1 and 1.06 to 1, and for right

hand corners between 1 to 1 and 1.09 to 1. As this is practically impossible, a compromise most be arrived at which will -give the most satisfactory results with a fixed' leverage ratio.

Whether a vehicle will corner more times to the right than to the left cannot be predicted. Such information. therefore, is not available for consideration when-the compromise is being arrived at, but the amount of full-lock work can be reasonably forecast.' A vehicle normally used on long-distance work uses full lock but seldom, whilst a lorry on local delivery work or a p.s.v., whilst rarely using absolutely full lock, very often comes near to it. But to provide two leverage systems, with such a small difference, wcatkl be both uneconomical and confusing from the service angle, so that the compromise resolves itself into a Choice

of a Mean figure to give satisfactory results for both classes of vehicle. On the hypothetical vehicle discussed, this would be about. 1,05 to 1.

When twin-front-axles were first incorporated in a chassis, they were steered by connecting the two axle steering arms by a drag link, the leverage being obtained by varying the length of the arms. Because of the unsuitability of the front-axle steering arm as a pivotal point for the second axle drag link, considerable interference occurred between the axles when the springs deflected. This interference was caused by the pivotal point being situated below the line C D E (Fig. 1) so that the condition indicated by the 'broken and full lines obtained. Furthermore, the constant rise and fall of the axles caused both pivotal centre and steering. arm to move, thus aggravating any undesirable condition.

The present-day form of linkage, shown diagramatically in Fig. 5, was evolved to eradicate the fault. it will be noticed that the system is arranged so that the leverage causes the wheels on the second axle to turn through a wider angle than these on the first axle..

A twin-front-axle chassis is subjects tit all the steering evils which beset the single-axle vehicle multiplied by two, to which the two further conditions of good steering, previously described, are added. These may not seem important on paper, but if one axle has any steering fault the accurate co-ordination built into the linkage of the twin axles will be upset, tyre wear will become excessive, and the steering doubly heavy.

It i,s imperative, then, that besides watching for the usual steering faults, the operator of. twin-front-axle vehicles must also be certain that the linkage between the steering box and the two axles is in no way distorted, unless he be prepared to accept 'the heavy steering and excessive tyre wear already mentioned.

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